Atherosclerosis is the main trigger of myocardial infarction, stroke and peripheral vascular disease, which remains the leading cause of death in the industrialized world. There is an unmet medical need for novel anti- atherosclerotic drug interventions. A critical roadblock in this endeavor is our current inability to fully understand pathogenic and regulatory pathways in atherogenesis. One of the important pathological features of atherogenesis is abnormal accumulation of smooth muscle-like cells, so-called synthetic smooth muscle cells (SMCs). Synthetic SMCs are proliferatory, migratory, secretory, inflammatory and apoptotic, and thus play critical roles in the initiation, progression and rupture of atherosclerotic plaques. We recently discovered that cyclic nucleotide phosphodiesterase 1C (PDE1C) is specifically expressed in synthetic SMCs of diseased vessels but not in normal vasculature and that proatherogenic stimuli angiotensin II (Ang II) and reactive oxidative stress (ROS) can activate PDE1C through ribosome S6 kinase (p90RSK). PDE1C activation is essential for mediating Ang II-induced lysosomal cholesterol accumulation and chemokine expression in synthetic SMCs. We also found that PDE1C is present in lysosomes and is likely involved in lysosomal destabilization and cholesterol accumulation, which subsequently induce ROS production and stress-induced cell apoptosis due to lysosome dysfunction. We will therefore explore the hypothesis that PDE1C acts as a novel critical positive regulator of various pro-atherogenic features of synthetic SMCs in the atherosclerotic process. By characterizing the functional relationship of PDE1C-regulated SMC pathogenesis in atherosclerosis in detail, we aim to elucidate the novel molecular mechanism of atherosclerosis development, and to develop novel therapeutic strategies for treating this disease. To achieve our goals and address our hypotheses we propose the following Specific Aims. In Aim 1 we will employ an array of biochemistry and cell biology approaches to determine how PDE1C regulates lysosomal function and cholesterol accumulation as well as inflammatory response in synthetic SMCs. In Aim 2 we will understand how PDE1C is activated by p90RSK and establish the biological link of p90RSK-mediated PDE1C activation in pathogenesis of synthetic SMCs. In Aim 3 we will determine the extent to which PDE1C deficiency attenuates atherosclerosis lesion formation and vascular pathologies in a well-established mouse model of atherosclerosis using global and SMC-specific PDE1C-knockout mice. We will also characterize the effects of disrupting p90RSK-mediated PDE1C activation by their binding-site peptide on vascular atherogenic remodeling. The innovative approaches and technologies we propose here will enable us to unveil the novel molecular regulatory mechanisms underlying synthetic SMC pathology in atherogenesis, to identify novel therapeutic targets, and to design new therapies to inhibit synthetic SMC pathologies given that PDE superfamily represents a highly attractive class of drug targets for the development of specific therapeutic agents.